1. Field of the Invention
The present invention relates to an apparatus for delivery of an ammonia dopant into a spectrometry system, such as an ion mobility or ion trap mobility spectrometer. Such spectrometers are commonly used for detection of narcotics, explosives or chemical weapons by law enforcement or military officials. This invention further pertains to an apparatus for use in the delivery of an ammonia dopant into a spectrometer system in which the apparatus can be transported and stored without the inherent costs and difficulties associated with the transport, storage and use of pure ammonia.
2. Description of the Related Art
A typical spectrometer of the type in which the present invention is used is an ion mobility spectrometer (IMS) or an ion trap mobility spectrometer (ITMS) used for detection and identification of low concentrations of chemicals. The presence of such chemicals may be used to indicate the presence of narcotics, explosives or chemical weapons to law enforcement officials.
IMS and ITMS systems detect and identify low concentrations of chemicals by measuring the time it takes an ionized sample to traverse a homogeneous electric field within an enclosed tube.
In IMS and ITMS, sample vapor or particulate matter of interest is first collected in a sample trap. In ITMS systems, the sample is then filtered through a semi-permeable membrane to extract unwanted dust and dirt. Sample molecules are then transported on a stream of carrier gas into a chamber in which the sample is ionized by, for example, corona discharge, atmospheric pressure photoionization, electrospray ionization, or introduction of a radioactive source. The ionized sample is then introduced into an enclosed tube in which a homogeneous electric field is present (in IMS the ionized particles must traverse a gating mechanism which only allows particles of specific polarity into the enclosure, while in ITMS, the particles are allowed to reach equilibrium and then are guided to the enclosure). The electric field in the enclosure drives the ionized sample through neutral molecules present within the enclosure to the opposite end of the enclosure, where a detector, such as a Faraday plate or mass spectrometer, measures the arrival of ionized species. The time measured between introduction of a sample and its arrival at the detector is directly proportional to the size of the ion and thus can be used to identify the presence of specific ions.
Interference in, and thus reduced efficiency of, an IMS or ITMS system may be created by ionization and subsequent detection of carrier gases. However, introduction of a low-concentration of a dopant may be utilized to accept the transfer of charges created during ionization of the carrier gas. Such an introduction may give rise to constant detector readings, which can be utilized for proper system calibration and proper system output. The carrier gas may be doped by flowing around a permeation tube, where a controlled concentration of a first dopant is added to the air stream. The permeation tube may be temperature-controlled, the temperature preferably being maintained by a thermostat to insure a constant flow of the dopant. This temperature control may be application of heat to the permeation tube by a heating component of the spectrometer.
For example, an IMS or ITMS system being utilized for the detection of alkaloids, such as narcotics, would be run in positive mode. Running of an IMS or ITMS system in a positive mode indicates the introduction of positively charged molecules into the carrier gas and sample during ionization. Sensitivity of such a device is enhanced through the introduction of an abundance of protons during ionization to ionize a higher percent of the sample of interest. However, such an introduction may ionize the carrier gas and yield a vast abundance of extraneous readings, leading to enhanced sensitivity at the cost of reduced data value. The introduction of a dopant charge transfer mediator with a proton affinity would serve to allow transfer of the positive charge to the dopant instead of the carrier gas, avoiding the ionization of the carrier gas. Thus, a single reading indicating the presence of the dopant is outputted with the continued increase in instrument sensitivity.
In searching for narcotics, spectrometers commonly are setup to detect alkaloids. In order to ensure the continued creation of a positive charge within alkaloids present in a sample, and to ensure the continued relative neutrality of carrier gas, it is important to select a dopant with a proton affinity lower than the narcotic, yet higher than the carrier gas. Ammonia's proton affinity is between that of common carrier gases, such as air or nitrogen, and that of alkaloids. It is common knowledge within the industry that ammonia suits these characteristics, and thus ammonia is the primary dopant used for detection of narcotics within an IMS or ITMS system.
The use of ammonia, however, creates a number of difficulties. Ammonia is a pressurized liquid at room temperature. When provided as a liquid, transportation is strictly controlled. Therefore, liquefied ammonia has typically been sealed in a permeation tube, and frozen until use. When used the permeation tube permits the controlled introduction of the dopant into a gaseous stream after permeation though a membrane between concentrated dopant and the gaseous stream. Ensuring a continuous flow rate from the permeation tube is sometimes problematic. Thus permeation tube construction can exacerbate these problems. IMS or ITMS systems commonly introduce the dopant into the system via introduction of the dopant into the carrier gas upstream from introduction of the sample. These limitations result in the permeation tube size being limited to 0.5 cubic centimeters, which typically limits the ammonia product's lifespan to six-weeks. As a further difficulty, ammonia is also very alkaline and reacts corrosively with all body tissues. Thus, additional safety measures must be abided by during transport of ammonia. In light of such difficulties, the use of ammonia as a dopant within a narcotic-detecting IMS or ITMS system is difficult.